<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8" />
		<meta name="viewport" content="width=device-width, initial-scale=1">
		<title></title>
		<style type="text/css">
			#box{
				width: 300px;
				height: 300px;
				background: gray;
				position: absolute;
				left: 0;
				top: 0;
			}
			#header{
				width: 100%;
				height: 50px;
				background: black;
				position: absolute;
				left: 0;
				top: 0;
			}
			#header span{
				width: 50px;
				height: 50px;
				background: yellow;
				color: black;
				position: absolute;
				right: 0;
			}
		</style>
	</head>
	<body>
		<div id="box">
			<div id="header">
				<span>B</span>
			</div>
		</div>
		<script type="text/javascript">
			var oDiv = document.getElementById("box");
			var oSpan = document.querySelector('#header span');
			var arr = [];
			oDiv.onmousedown = function(evt){
				//获取事件对象的兼容
				var e = evt || window.event;
				//获取事件源的兼容
				var target = e.target || e.srcElement;
				//事件委托
				if(target.id == 'header'){
					//鼠标相对于事件源元素的x、y坐标
					var disX = e.offsetX;
					var disY = e.offsetY;
					//#box初始位置
					arr.push({"left" : oDiv.offsetLeft,"top" : oDiv.offsetTop});
					//鼠标移动事件
					document.onmousemove = function(evt){
						var e = evt || window.event;
						var oHtml = document.documentElement;
						var left = e.pageX - disX;
						var top = e.pageY - disY;
						//控制#box不脱离浏览器边框
						if(left <= 0){
							left = 0
						}else if(left >= oHtml.clientWidth - oDiv.offsetWidth){
							left = oHtml.clientWidth - oDiv.offsetWidth
						}
						if(top <= 0){
							top = 0;
						}else if(top >= oHtml.clientHeight - oDiv.offsetHeight){
							top = oHtml.clientHeight - oDiv.offsetHeight;
						}
						oDiv.style.left = left + 'px';
						oDiv.style.top = top + 'px';
						//随时记录#box坐标,用于原路返回
						arr.push({"left" : oDiv.offsetLeft,"top" : oDiv.offsetTop}); 
					}
					//鼠标松开事件
					document.onmouseup = function(){
						document.onmousemove = null;
					}
					//禁止鼠标拖动
					document.ondragstart = function(){
						return false;
					}
				}
			}
			//span元素新增监听事件,将#box原路返回
			oSpan.addEventListener("click",function(){
				var index = arr.length - 1;
				var timer = setInterval(function(){
					if(index == -1){
						clearInterval(timer);
						arr = [];
					}else{
						oDiv.style.left = arr[index].left + 'px';
						oDiv.style.top = arr[index].top + 'px';
						index --;
 					}
				},20)
			},false);
		</script>
	</body>
</html>